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Kind signatures are handled by `TcTyClsDecls.tcTyClsDecl` together with all other type and class declarations. Within class declarations, we invoke the functions recursively to process associated types.

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Instances of indexed types are type checked by `TcTyClDecls.tcIdxTyInstDecl`; i.e., the same functions that performs their kind checking. Kind checking and type checking of instances of indexed types can be combined, as we don't need to worry as much about recursive dependencies as we have to for standard type declarations. In particular, the kinds of indexed types are declared by their signature and we don't have to compute any recursiveness information, as we never know whether we reached a fixed point for open types. (Hence, we conservatively assume indexed types to be always `Recursive`. This is safe as they are implicit loop breakers due to to implying coercions.)